1. Field of the Invention
The present invention relates to a method of welding cylindrical bodies used when manufacturing a can containing, for example, beverage.
2. Description of the Related Art
Generally, as manufacturing means of such can, both edges of a metal plate are overlaid and welded into a cylindrical form, and lid plates are attached to both sides of the opening of the cylindrical body.
Such welding method was conventionally disclosed, for example, in Laid-open Japanese Patent Publication No. 58-25886 and Laid-open Japanese Patent Publication No. 4-49794(U.S. Pat. No. 2,524,047). In these publications, the welding current waveform is controlled by chopping in multiple pulse forms by a chopping mechanism in every half cycle of welding frequency, and welding is done by the current waveform under chopping control. In these welding methods, as compared with the known method of welding by forming current waveforms by on/off switching control, generation of splashes in the nugget between overlaid portions of a metal plate is suppressed, and a cylindrical body excellent in strength is obtained.
In such welding methods, however, since chopping current waveforms of multiple pulse forms are formed in every half cycle of welding frequency, the chopping mechanism is required to send and receive multiple signals. It is also required to check every one of these signals. Hence, in relation to the response time, the welding speed is slow. In particular, since it is necessary to control the rising straight line of current waveform by chopping with plural signals, the rising speed of current waveform is slow. As a result, the welding speed is low.
Recently, in order to lower the manufacturing cost of cans, it is demanded to increase the manufacturing speed of cylindrical bodies, that is, the welding speed. For example, hitherto, the speed as about 500 pieces/minute, but it is recently requested from the users to enhance the speed to about 1000 to 1200 pieces/minute. Such request is not satisfied if welding is done by such chopping current waveform.
The present inventor previously proposed a welding method for cylindrical bodies at high speed (Japanese Patent Application No. 7-341741). Its basic constitution is shown in FIG. 11, in which, in every half cycle 1/2f of welding frequency f, the welding current waveform comprising a rising straight line A rising up to a specified rising current set value S1, a pause straight line B extending from its upper end, a drop straight line C dropping from its rear end to a specified current drop set value S2, a drop pause straight line D extending from its lower end, and at least a rising straight line A1 and a pause straight line B1 consecutive to this drop pause straight line D is formed by the conventional on/off switching control without resort to chopping control. Consequently, the current set value S1 of the welding current waveform is set constant and the current drop set value S2 is set constant. Further, by individually varying the duration of time T21, T22 of each pause straight line B, B1, the drop pause straight line D is set at specified time T23.
By welding the overlaid portions of a cylindrical body while repeating such welding current waveform, small nuggets are formed between the overlaid portions, without generating splashes, favorable in a uniform size and at a uniform small pitch interval. Moreover, without forming blow holes or cracks, the cylindrical body is excellent in strength. In the case of cylindrical bodies used in general beverage cans (cylindrical length being 70 to 140 mm), it is possible to weld at high speed of about 1000 to 1200 pieces/minute.
In this welding method, however, although the cylindrical body can be welded at high speed, the following inconvenience was discovered in actual work. That is, when welding cylinders, it is necessary to vary the welding current waveform depending on their kind and material. In this welding method, to vary the welding current waveform, it is necessary to set again the time T21, T22 of each pause straight line B, B1, time T23 of drop pause straight line D, rising current set value S1, and current drop set value S2 multiple times on every occasion. Therefore, the working efficiency is poor.
It is hence an object of the invention to present a welding method capable of forming small nuggets of uniform size between overlaid portions of a cylindrical body favorably at small pitch intervals to obtain cylindrical bodies of excellent strength, lowering the cost by welding the cylinders at high speed, and enhancing the working efficiency by decreasing the number of times of setting when changing the welding current waveform depending on the kind and material of cylinders.